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Henry K. Ngugi and James R. Schupp

question as to whether the thinner can transmit the fire blight-causing bacterium Erwinia amylovora (Burrill) Winslow et al., thereby increasing disease risk. The manufacturer of the rotating string thinner reported that in Germany, the incidence and

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Gayle M. Volk, Christopher M. Richards, Adam D. Henk, Ann A. Reilley, Patrick A. Reeves, Philip L. Forsline, and Herb S. Aldwinckle

susceptibility was determined based on the presence of pycnidia on seedling tissues 8 weeks after inoculation with basidiospores of Gymnosporangium juniperi-virginianae . Greenhouse plants were inoculated with Erwinia amylovora strain Ea273. The two youngest

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J.R. Schupp, D.A. Rosenberger, T.L. Robinson, H. Aldwinkle, J. Norelli, and P.J. Porpiglia

Three studies were conducted to evaluate the effect of post-infection sprays of prohexadione-calcium on the severity of naturally occurring fire blight infections on 3- and 4-year-old 'Gala' apple trees on blight-susceptible or blight-resistant rootstocks. Although post-infection prohexadione-calcium reduced the dry weight of fire blight strikes removed by pruning in one commercial orchard site, this treatment did not reduce mortality of young 'Gala' trees on M.9 or M.26 rootstocks, and did not reduce the incidence of scion or rootstock cankers on any of the rootstocks tested. We conclude that post-infection treatment with prohexadione-calcium is of no practical value in reducing fire blight symptoms on apple. Our results suggest that resistant apple rootstocks will be very valuable in increasing orchard survival in a fire blight epidemic.

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David M. Hunter, Frank Kappel, Harvey A. Quamme, and W. Gordon Bonn

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David M. Hunter, Frank Kappel, Harvey A. Quamme, and W. Gordon Bonn

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Gayle M. Volk, Christopher M. Richards, Ann A. Reilley, Adam D. Henk, Patrick A. Reeves, Philip L. Forsline, and Herb S. Aldwinckle

presence of pycnidia on seedling tissues after inoculation with basidiospores of Gymnosporangium juniperi-virginianae . Fire blight resistance was determined by resistance to Erwinia amylovora strain Ea273 inoculation greenhouse plants. Shoots were

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Richard L. Bell, Tom van der Zwet, Roland C. Blake, Craig K. Chandler, and Joseph C. Scheerens

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Christopher S. Walsh, Julia M. Harshman, Anna E. Wallis, Amy Barton Williams, Michael J. Newell, and George R. (G.R.) Welsh

and pear stocks California State Hort Commun. Monday Bul. 4 145 149 Steiner, P.W. 1989 Predicting apple blossom infections by Erwinia amylovora using the MARYBLYT model Acta Hort. 273 139 148 Van der Zwet, T. Kiel, H. 1979 Fire blight a bacterial

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R.L. Bell, T. van der Zwet, and Roland C. Blake

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Alex L. Shigo

der Zwet for samples of pear infected with Erwinia amylovora; and Kevin Smith and Philip Larson for valuable discussions on the subject.